The Z(4430), a new subatomic particle that provides smoking gun evidence for a four-quark meson

Researchers at the University of Hawaii at Manoain the Physics and Astronomy Department workingon the Belle experiment in Tsukuba, Japan reportedthe discovery of a new class of subatomic particle in the April 11thissue of Physical Review Letters, the leading US physics journal.

In 1964, Murray Gell-Mann proposed that the subatomicparticles seen in nature are comprised of differentconfigurations of fundamental constituents thathe called quarks. In Gell-Mann's scheme, quarks formparticles in two ways: three quark combinationsproduce "baryons," particles like the proton and neutron thatform the nuclei of atoms in most matter,while quark-antiquark combinations form "mesons," such asthe pi-meson and K-meson.

For the four decades that have elapsed since the birth of the quark idea, Gell-Mann's three-quark baryon quark-antiquark meson description has sufficed to explain all observed particles. Recently, however, researchers in the Belle experiment in Japan have found a meson that that defies classification as a simple quark-antiquark state. This meson, called the Z(4430), is seen to decayinto an electrically neutral psi' meson --a well establishedcharmed-quark anticharmed-quark state-- plus a charged pi meson.The psi' among the decay products indicates that the parentZ(4430) must contain a charmed-anticharmed quark pair. However,this quark combination is necessarily electrically neutral. Since the Z(4430)is electrically charged it must contain an additionalquark-antiquark pair (up-quark antidown-quark for the Z+(4430) anddown-quark antiup-quark for the Z-(4430).

Four-quarkmesons have been proposed by theorists and a numberof candidate "tetraquarks" have been identified inexperiments, most notably the X(3872), A meson thatwas found by UH researchers in 2003. However, prior tothe Z(4430), all such candidates have beenelectrically neutral, which allowed the possibility for a quark-antiquark classification. This "out" does notexist for the charged Z(4430), which appearsto be smoking-gun evidence for a tetra-quark meson state.

The Physical Review Letter on the Z(4430) is the result of work by ProfessorStephen Olsen (University of Hawaii) and Sookyung Choi (GeyongsungUniversity) in Korea and the Belle experimental collaboration.

Other University of Hawaii participants in Belle includefaculty members Tom Browder, Michael Jones, Mike Peters, Gary Varner,postdoctoral fellows Herbert Hoedlmoser and Li Jin, graduatestudents Hulya Guler, Kurtis Nishimura, Jamal Rorie and Himansu Sahoo. The work in the Belle experiment, an international collaboration of physicists from countries in Asia, America, Europe and Australia is supportedby the US Department of Energy. The University of Hawaii group also plansto participate in the Super KEKB factory upgrade that will startoperation in 2012 in Tsukuba, Japan.